Various standard specifications regarding mobile communication systems have been developed by 3GPP (The 3rd Generation Partnership Project). In a 3G (3rd generation) system or an LTE (Long Term Evolution) system, a maximum rate used for communications can be controlled for each user. For example, the maximum rate for a user is determined according to the type of user's subscription or a service condition and provided from a core network to a base station. This maximum rate is refereed to as an AMBR (Aggregate Maximum Bit Rate). The AMBR can be found in 3GPP, TS 24.301, V8.4.0 (2009-12).